In Part 1 we set the stage for recognizing that much beyond reduced visual acuity, amblyopia is a developmental disorder of spatial vision that represents impaired perceptual learning. Perceptual learning is involved in just about any visual task you can think of, as noted in this excellent review by Lu et al. We also noted the link between crowding and dyslexia, as resurrected by Levi and colleagues. A direct link between crowding and dyslexia stems from the superb chapter by Janette Atkinson in a gem of a book on Vision and Visual Dyslexia. You’ve probably encountered young children who are familiar with all their alphabet letters, but have a difficult time reading the letters on a conventional eye chart. When we lose their interest or attention we tend have an assistant point to each letter, or we might box off one line, perhaps even individual letters, which reduces their confusion or uncertainty about where to look.
In other words, crowding or losing focus in clutter is a normal visual response for pre-schoolers. Atkinson quantified this effect with Cambridge Crowding Cards, in which the letter to be identified was surrounded by other letters. A crowding ratio is based on the size of the smallest crowded letter identified : size of the smallest single letter identified, and made the following key observations.
1) Contour interactions, perceptual effects, and task difficulty may all increase crowding in pre-school children, but these effects tend to diminish in normally developing children by age six or seven to approach normal adult levels.
2) Children who had uncompensated hyperopia in infancy appeared to be more visually delayed in terms of crowding even they though had normal visual acuity on single letter tasks.
3) The crowding scores for untreated hyperopes at age five were much like those of the four year-old controls and of the four year-old treated hyperopes. This prompted Atkinson to comment that a number of children are starting school and attempting pre-reading tasks at a visual disadvantage, with subtle delays in development, including perceptual or cognitive visual immaturity, despite good isolated letter visual acuity. This is also consistent with the work of Rosner and Rosner linking lags in visual perceptual skills with uncompensated hyperopia, particularly at younger ages.
4) Crowding effects tended to be more marked in older children experiencing dyslexia.
Recent research tends to support Atkinson’s original notion that there is a common link between the visual immaturities that lessen in the course of normal visual development, and the persistence of these immaturities or delays in development in the presence of strabismus and amblyopia. We can further state with regard to crowding that the delays in the amblyopic eye extend to abnormalities in both space and time, and that vision in the “non-amblyopic” eye is not normal with regard to contour interaction as compared to control eyes in patients without amblyopia. We may soon have iPad applications that make it easier to quantify these effects across a broad population of children who have amblyopia along with multiple impairments, and for children at risk for dyslexia who persist with visual crowding effects.
With that background we can address the question originally posed: Is the amblyopic eye learning disabled? Given the research reviewed from Atkinson onward, and the recognition in vision science that amblyopia is a developmental disorder of spatial vision ameliorated through perceptual learning, it is reasonable to view the patient with inadequately treated amblyopia as having a fundamental disability in learning through visual channels. So the answer is yes: the amblyopic eye is learning disabled.
There is a misnomer that a child with amblyopia is referred to as having a “lazy eye”, but perhaps that’s linked to the misperception that a child with learning disabilities is “lazy”, which calls to mind the following cartoon graphic.
If this premise is correct, we could be doing a better job collectively of supporting patients with amblyopia by arranging conditions for improved perceptual learning. This thinking is reflected in the design of vision therapy activities targeted for use in the PEDIG studies, intended for use in ATS-VT downloadable here. These procedures do a nice job of addressing visual crowding and a series of ocular motor skills, incorporating binocular integration. The only missing element appears to be contrast sensitivity matching between the two eyes as suggested by the Hess lab, though that is a more recent development.
Perhaps one day the PEDIG ophthalmology sites will be able to recruit enough subjects for the study to proceed. In the interim, optometric practices will continue to treat patients with amblyopia through binocular perceptual learning paradigms. It is reasonable to anticipate that science will catch up to our clinical success and, in turn, our clinical success will be further informed by vision science.